What change occurs to the focal spot when using high mag mode?

When using high magnification mode in fluoroscopy, there is indeed a change in the focal spot's location due to the adjustment in the focusing lenses used in the system. In high magnification mode, the focusing lens voltage increases, which leads to a greater convergence of the electron beam directed at the input phosphor. This adjustment allows the focal spot to shift closer to the input phosphor rather than the output phosphor, thereby enhancing image resolution and detail.

This shift closer to the input phosphor helps in maintaining image quality as it compensates for the increased distance caused by magnification, allowing for a more focused image while still providing adequate brightness. It is important in such imaging techniques to ensure that the resulting images remain clear and sharp, which is crucial for diagnostic purposes in fluoroscopy.

The other choices do not accurately reflect the mechanics of high magnification mode as they do not correctly describe how the focal spot and lens voltage interact during this particular adjustment in fluoroscopic imaging.